docs: def getActiveAxes(self):
          """
          Return a list of names of axes which are not zero
          ::
          
              >>> l = Location(pop=1, snap=0, crackle=1)
              >>> l.getActiveAxes()
              ['crackle', 'pop']
          """
          names = sorted(k for k in self.keys() if self[k]!=0)
          return names